As devices become smaller and integration density increases, reactive ion etching (RIE) has become a key process in anisotropic etching of semiconductor feature. RIE or ion-enhanced etching works by a combination of physical and chemical mechanisms for achieving selectivity and anisotropicity during the etching process. Generally, plasma assisted etching operates in the milliTorr range and above. Generally, three processes compete with each other during plasma etching; physical bombardment by ions, chemical etching by radicals and ions, and surface passivation by the deposition of passivating films. In some applications, for example, high density plasmas (HDP) having a higher density of ions and operating at lower pressures have been increasing used in etching processes.
One RIE process also referred to as plasma ashing uses an oxygen plasma source gas to accomplish the removal of photoresist or polymeric residues following an etching process. One problem area in plasma ashing relates to the removal of photoresist masks following an ion implantation process, for example to form source and drain regions. Typically a silicon oxide layer is formed over the silicon at the source and drain regions prior to carrying out a high dose implant (HDI). During the HDI, the photoresist is subjected to high energy ions that induce cross-linking reactions to harden an upper shell of the photoresist. Subsequent ashing processes using only oxygen are generally ineffective for removing the hardened photoresist, requiring the use of fluorine containing plasma source gases to remove the hardened photoresist and polymer residues.
One problem with a fluorine/oxygen plasma etching chemistry is that silicon over the doped silicon regions, for example the source/drain regions tends to be removed thereby altering the electrical operation of a MOSFET device. For example drain current saturation at a given applied gate voltage is altered, for example lowered. In addition, parasitic leakage, for example junction leakage due to decreased junction depth is increased detrimentally altering electrical functioning of the device.
Another problem with prior art processes, is that the use of fluorine containing plasmas in photoresist removal processes tends to preferentially etch through the oxide layer which is formed over the source and drain regions to moderate the ion implantation energies and protect the underlying silicon. As a result, plasma etching damage to the underlying silicon frequently results including etch away a portion of the silicon.
There is therefore a need in the semiconductor device processing art to develop an improved method for etching photoresist layers following a silicon doping process to avoid plasma etching damage including loss of doped silicon areas thereby improving device reliability and process wafer yield.
It is therefore an object of the invention to provide an improved method for etching photoresist layers following a silicon doping process to avoid plasma etching damage including loss of doped silicon areas thereby improving device reliability and process wafer yield while overcoming other shortcomings and deficiencies of the prior art.